The present invention relates to novel unitary composite materials comprising a lightened thermoplastic polymer layer, a setting polymer layer, and a homogeneous intermediate layer. The invention also relates to the manufacturing processes.
In contrast to "expanded thermoplastics", which are cellular products of low density, the term "lightened thermoplastic" is used herein to mean the category of cellular products whose mechanical properties are closest to those of the corresponding compact product. The density of the lightened thermoplastics of the invention is greater than 0.15 g/cm.sup.3.
Composite materials consisting of expanded materials reinforced with thermosetting polymers are known, but in all cases, when it is technically possible to produce such materials, the resultant composites have a heterogeneous structure. These materials are obtained in two ways: one method involves direct crosslinking of the thermosetting resin on the expanded material, and the other involves adhesion; that is to say, the use of an intermediate adhesive material between the two main constituents.
It is known to use polyester resins for the direct preparation of protective layers on parts made of expanded polystyrene. As indicated in French Pat. No. 1,480,638, this direct crosslinking process has a major disadvantage in that the expanded material is attacked by the customary polyester resins. In fact, the free monomer contained in the thermosetting resin acts as a solvent and attacks the cells of the expanded material, resulting in the destruction of this expanded material and thus preventing the production of an industrially acceptable composite material. To overcome this disadvantage, it was proposed in French Pat. No. 1,085,567 to apply to the forms, prior to application of the thermosetting resin, a layer of a substance which firstly does not attack the expanded material and secondly is not attacked by the thermosetting resins. Apart from the fact that this process is rather impractical and slow, the product obtained lacks unity due to the presence of this intermediate foreign substance. This same lack of unity is found again when an adhesive, which is also an intermediate substance, is used to combine an expanded or lightened thermoplastic material with a thermosetting material. This lack of unity leaves the interfaces sensitive to all the phenomena capable of causing separation. It can be said that the use of an intermediate adhesive means makes it possible to obtain not a unitary composite, but simply a juxtaposition of plastic elements, the final structure of which is heterogeneous.
Attempts have been made to overcome, at least in part, the disadvantages of the previous techniques in French Pat. No. 1,480,638. According to the said patent, a polyester resin is polymerized directly on an expanded polystyrene material after the major part of the monomer which is a solvent for the polystyrene, contained in the polyester, has been replaced with an N-methylolurea allyl ether which is a nonsolvent for the polystyrene. But here again, only a nonunitary and hence heterogeneous composite can be obtained; it only being possible for the bonding between the two components to be mechanical. In fact, since the polyester resin no longer possesses a means of acting directly on the polystyrene itself, it can only infiltrate and crosslink in the pores of the expanded material, thereby causing only mechanical attachment without intimate bonding, which results in the production of a product having limited mechanical properties.